One type of filter apparatus for filtering suspended solids includes a tank having an inlet and an outlet, a filter frame positioned between the inlet and outlet, and filter material supported on the filter frame. During a filtering operation, liquid effluent is passed through the filter material such that the filter material filters out suspended solids from the effluent stream. The filtered solids are deposited on the filter material while the filtered liquid stream is discharged from the tank as effluent.
To remove the filtered solids from the filter material, it is known to provide the filter apparatus with a centrifugal pump that is connected to a suction head positioned adjacent a surface of the filter material. The centrifugal pump is operated to draw filtered liquid through the filter material in a backwash or reverse direction. The backwash flow that results acts to dislodge the filtered solids from the filter material.
In practice, however, filtered solids continue to accumulate within the filter material despite successive backwash operations. Some of the filtered solids become so deeply embedded in the filter material that the backwash flow is ineffective in dislodging these solids. Accordingly, the friction losses across the filter material begin to rise, which causes a corresponding reduction in the backwash flow rate generated by the pump. Further, as a result of the reduction in backwash flow rate, solids accumulate within the filter material at an increased rate. Thus, the effectiveness of the backwash operation diminishes with time and as solids continue to accumulate within the filter material. If the net positive suction head for the pump is allowed to continue to drop, the pump experiences cavitation and/or loses suction altogether. Either condition is undesirable because it causes permanent damage to equipment (e.g., the pump impeller and seals), requires operator attention, and interrupts the filter operation.
In a second cleaning step, a high-pressure liquid spray is directed from one or more spray nozzles to the surface of the filter material. The high pressure spray penetrates the surface, impacts the deeply embedded solids, and dislodges these solids from the filter material.
However, in order for the spray to be effective, the tank must be drained of sufficient liquid volume so that the spray nozzles are above the liquid level. This typically requires the filtering operation to be stopped for the duration of the spray operation. Therefore, to minimize filter downtime, the spray operation is initiated only when solids accumulation on the filter material is so great that the backwash flow is rendered ineffective, or alternatively, when it is convenient to stop the filtering operation.